1. Field of the Invention
This invention relates generally to an improved process for the production of cardanol from cashew nutshell liquid (CNSL). The process is distinguished by the fact that the cardanol thus obtainable contains fewer secondary products than the usual, commercially available cardanol.
2. Background Information
Phenalkamines are still a relatively young class of epoxy resin curing agents. They are products of the reaction (condensation products) of cardanol (I), which, chemically, is a C15 alkylphenol and a major constituent of the oil obtainable from cashew nutshells (CNSL=cashew nut shell liquid), with aliphatic (primary or secondary) amines and formaldehyde.
Information on the class of phenalkamines can be found in the following publication: Zhishen Dal et al., “Phenalkamines: Multipurpose Epoxy Curing Agent”; Cardolite Corporation, Newark, N.J., USA; Reprint SPI-ERF Conference, September 1994.
It is known that crude CNSL predominantly contains a compound known as anacardic acid (II). The distillation of CNSL in the presence of acid gives a composition which mainly contains cardanol and, as a secondary product, cardol (III), cf. for example U.S. Pat. Nos. 6,262,148 and 6,229,054. This is consistent with applicants' own studies, according to which the distillation of crude CNSL gives a composition which mainly contains cardanol and, as a secondary product, cardol plus small quantities of 2-methyl cardol and anacardic acid.

The cardanol/cardol mixture obtained in this way has three technical disadvantages:                Its production by distillation from crude CNSL involves losses of value because part of the cardanol is lost through polymerization so that, ultimately, the yield of cardanol in the distillate is only 50-60%.        The initially pale yellowish cardanol/cardol mixture changes during storage, rapidly turning brown in color. This unwanted change in color is attributed to the presence of cardol.        Products ensuing from the cardanol/cardol mixture also undergo unwanted changes in color during storage.        
It has also been proposed to improve the color stability of cardanol/cardol mixtures by reducing the cardol content by special measures. To this end, it has been proposed first to react the cardol present in the CNSL largely selectively with aldehydes, amines or bases and hydroxides of alkali and alkaline earth metals and then to distil off the unreacted cardanol. Particulars of these processes for the production of cardanol with improved color stability can be found in GB-A-2,152,925, GB-A-2,066,820 and U.S. Pat. No. 4,352,944.